Braconid(Hymenoptera:Braconidae) fauna in rice ecosystems of central Kerala

Abstract

Rice (Oryza sativa L.) an annual grass, serves as the staple food for over 65 per cent of the world's population. It supports a rich and diverse fauna of insect pests and natural enemies, and thus maintains a sustainable ecosystem. Biodiversity in agricultural land has received growing attention as it plays a significant role in agro ecosystem function by keeping the pest populations under check. Any imbalance in the diversity generally causes pest outbreak. The over-reliance on insecticides is documented as one of the main reasons for loss of diversity of beneficial organisms like parasitic hymenopterans. Among hymenopteran parasitoids, Braconidae (Hymenoptera: Ichneumonoidea), the second largest parasitoid family, plays a vital role in controlling insect pests in rice ecosystem. They comprise primary or secondary parasitoids that exhibit a broad host range and gregarious mode of parasitism, making them valuable ally in biological control programs targeting lepidopteran insect pests of rice. No concerted effort has been made on documenting hymenopteran parasitoids in rice ecosystems of Kerala. The present study entitled "Braconid (Hymenoptera: Braconidae) fauna in rice ecosystems of central Kerala", was carried out during 2022-23 with the objective to document the braconid fauna in rice ecosystems; assess the relative diversity of braconids under different crop management practices and study the influence of weather parameters on the population. A purposive survey was carried out for one crop season (mundakan) from December to May in order to collect parasitoid samples from rice ecosystems. Samples were collected from two rice fields; one represented an ecosystem managed as organic and the other, managed in accordance with the package of practices recommendations, Kerala Agricultural University (POP, KAU) selected in Thrissur and Palakkad districts. Four different methods, viz., yellow pan traps placed on bund, yellow pan traps erected in field at canopy level, net sweeping and host rearing were employed to collect the samples. The study recorded 413 braconid specimens belonging to 15 subfamilies and 29 genera. The subfamilies included Alysiinae, Aphidiinae, Braconinae, Cheloninae, Euphorinae, Gnamptodontinae, Homolobinae, Hormiinae, Macrocentrinae, Meteorinae, Microgastrinae, Opiinae, Orgilinae, Pambolinae and Rogadinae. Bracon, Stenobracon, genus nr. Scutibracon, Tropobracon, Amyosoma, Aspidobracon, Leiophron, genus nr. Townesilitus, Gnamptodon, Homolobus, Hormius, Macrocentrus, genus nr. Zele, Apanteles, Glyptapanteles, Dolichogenidea, Exoryza, Cotesia, Opius, Phaedrotoma, Indiopius, genus nr. Diachasmimorpha, Phaenodus, Aleiodes, Binodoxys, Promicrogaster, Chelonus, Orgilonia and Aphaereta were the genera recorded in the present study. The study investigated the abundance and distribution of braconids in rice ecosystems under different crop management practices. The most abundant subfamily was Microgastrinae, which made up 55.1 and 54.8 per cent of the total population respectively in Thrissur and Palakkad districts. In Thrissur district, 91 braconid specimens under six subfamilies were recorded from organically managed field and 136 braconid specimens under five subfamilies from field under POP, KAU. In Palakkad district, 84 braconids belonging to 12 subfamilies were recorded from the organic field and 102 braconids under eight subfamilies from field under POP, KAU. Certain subfamilies, viz., Alysiinae and Rogadinae in Thrissur district, and Aphidiinae, Euphorinae, Gnamptodontinae, Homolobinae, Hormiinae and Pambolinae in Palakkad district were collected exclusively from field under organic management. Similarly, Cheloninae in Thrissur district, and Macrocentrinae and Meteorinae in Palakkad district were solely collected from field under POP, KAU. The average number of braconids collected per day was higher in yellow pan traps placed on bund (3.60 ± 2.719) followed by yellow pan traps erected in field at crop canopy level (2.26 ± 2.787), and sweep netting (0.74 ± 0.991). Comparison of braconid populations in rice ecosystems under various crop management practices showed no significant difference in the mean population size. However, diversity indices under different systems differed amongst fields. In Thrissur district, the braconid fauna was found to be more diverse and evenly distributed in rice field managed as per POP, KAU, as evidenced from higher Simpson's diversity index, Shannon-Wiener index, and Pielou's evenness index compared to organic field. In contrast, Margalef’s index indicated that the field had a higher richness. However, compared to POP, KAU managed field, organic field had greater diversity, richness and evenness indices in Palakkad district. A comparison of ecosystems using beta diversity analysis indicate that there is less similarity (31.6%) between fields in the Palakkad district and a moderate level of similarity (50%) between fields in the Thrissur district. The relationship between weather parameters and the population dynamics of braconid fauna was studied. In Thrissur district, significant positive correlation was found with bright sunshine hours of the previous week, while, significant negative correlations were established with evening relative humidity of the previous week and minimum temperature of the previous day. In Palakkad district, the braconid population and wind speed of the previous day showed strong positive correlation, while, morning relative humidity of the previous week showed a negative correlation. The research study revealed a rich diversity of braconid fauna in the rice ecosystems of central Kerala. It also revealed strong relationship between environmental factors and the population dynamics of braconids in rice ecosystems as well as the role of crop management practices in the diversity of braconid fauna. Further studies on the role of braconid parasitoids in managing insect pests of rice with host-parasitoid relationship, impact of climate change on distributpion and abundance, and conservation measures for long term plant protection may help in the development of more sustainable and efficient pest management strategy in rice ecosystems.